The present invention relates to a rolling mill system for hot strips, and more particularly to a method and an apparatus for joining hot strips by which the hot strips can be joined to each other in a short time for continuous rolling, when the hot strips are rolled by using a train of roughing mill stands (roughing train) and a train of finishing mill stands (finishing train).
There is a very strong demand for realizing continuous finish-rolling in hot rolling mill systems of metals to achieve an improvement in productivity, a higher degree of quality, and automatization of operations. The key technique for that purpose resides in joining of bars to each other. If the joining of bars is not completed in a short time, the traveling distance of a joining machine would be so long as to make realization in practical use difficult when the joining machine is of the traveling type, for example. On the other hand, when the joining machine is of the fixed type, a quite large-scaled looper would be required to accumulate bars because the bars are usually as thick as 30 to 50 mm.
As a method of joining bars to each other, many techniques such as of the electrically heating type, gas heating type or solvent type have been proposed in the past. However, since those techniques have their merits and demerits, any of them has not yet been successfully put into practice. The severest one of those demerits is related to the joining time as suggested above. More specifically, the time required for joining bars is too much; i.e., it takes 20 to 30 seconds at minimum, including preparation before start of the joining, removal of swellings due to pressing, etc. Another problem is that joining conditions are delicate and the quality of the joining is affected by slight differences in the conditions. Therefore, the technique capable of surely joining bars to each other in a short time is essential to realize continuous rolling in a practicable manner.
For that reason, various methods of mechanically joining bars rather than those using electricity or gas which are time consuming, have been tried and proposed. For example, JP, A, 51-137649 discloses a method of placing bars in partly overlapped relation and pressing the bars. JP, A, 51-130665 and 60-102207 disclose a method of cutting bars obliquely relative to the direction of thickness and overlapping the cut surfaces of both the bars, followed by rolling to join the bars together.
However, the above conventional methods have suffered from the following problems.
First, with the prior art disclosed in JP, A, 51-37649, fresh surfaces which become joined surfaces are less created because of a small relative slide between both the bars. The experiment has proved that the joining of the bars is not satisfactory in quality.
Secondly, with the prior art disclosed in JP, A, 51-130665 and 60-102207, it is difficult to obliquely cut the bars by a usual guillotine cutter. This difficulty is further increased at a larger oblique cutting angle. Such an oblique cutting is feasible by using a milling cutter or the like, but this step takes a too much time. Even if the bars can be obliquely cut, the joining force of the bars is so weak that they are likely to disconnect upon slight extent of bending, tension or oscillation when rolling is made with overlapping the cut surfaces of both the bars, because scales on the joining surface are not sufficiently crashed and removed.